In industrial measurement and automation technology, inline measuring devices utilizing a vibration-type measurement-transducer are often used for the highly accurate registering of measured, process variables of media flowing in lines, especially pipelines, and especially for the registering of flow-dynamic and/or rheological, measured variables. Such measurement transducers include at least one measuring tube, which is in communication with the medium-conveying pipeline and which vibrates during operation. Construction, operation, and ways of applying such vibration-type measurement-transducers are described comprehensively and in detail in U.S. Pat. Nos. 4,127,028, 4,524,610, 4,768,384, 4,793,191, 4,823,614, 5,253,533, 5,610,342, 5,796,011, 5,705,754, 6,006,609, 6,047,457, 6,168,069, 6,314,820, 6,352,196, 6,374,478, 6,397,685, 6,450,042, 6,487,917, 6,516,674, 6,519,828, 6,523,421, 6,598,281, 6,666,098, 6,698,644, 6,711,958, 6,769,163, 6,851,323, WO-A 03/048693, WO-A 05/050144 or in the not pre-published, U.S. patent application Ser. No. 11/242,803 of the present assignee.
As is known, vibration-type measurement-transducers serve in conjunction with a measuring device electronics connected thereto for producing, in the medium instantaneously conveyed in the at least one measuring tube, reaction forces appropriately corresponding with the process variable to be measured, such as e.g. Coriolis forces corresponding to a mass flow, e.g. a mass flow rate, inertial forces corresponding to a density, or frictional forces corresponding to a viscosity, etc., and to produce, derived therefrom, a measurement signal appropriately corresponding with the process measured variable, for example the particular mass flow, the particular viscosity and/or the particular density, of the medium. The at least one measuring tube of the measurement transducer is, for such purpose, usually medium-tight, especially pressure-tight, and, most often, also installed durably into the course of the pipeline conveying the medium. For instance, the measuring tube can be installed into the pipeline by means of appropriate flange connectors. For the oscillatable holding of the at least one measuring tube, a tubular or frame-like support element, for example of steel, is provided. The support element is, most often, embodied much more stiffly than the measuring tube and is mechanically coupled to the measuring tube at its inlet and outlet ends. For example, the support element is affixed directly to the measuring tube. The support element can, as is usual for measurement transducers of such type and as is also perceivable, without more, from the above-cited state of the art, be completed to form a measurement transducer housing, which encases the transducer, by means of appropriately externally applied covers, such as e.g. tube-covering caps or laterally mounted sheets, or the support element can itself be embodied as the measurement transducer housing.
For driving the at least one measuring tube, measurement transducers of the described kind additionally include an exciter mechanism eclectically connected with the relevant measuring device electronics and having an oscillation exciter, especially an electrodynamic or electromagnetic one, mechanically interacting with the measuring tube. During operation, the exciter mechanism is so actuated by the measuring device in suitable manner by means of corresponding exciter signals, that the measuring tube executes, at least at times, vibrations, especially bending oscillations and/or torsional oscillations. Additionally present is a sensor arrangement delivering oscillation measurement signals. At least in the case of use of the measurement transducer as a Coriolis mass flow measurement transducer, the sensor arrangement includes at least two sensor elements separated from one another and reacting to measuring tube vibrations of the measuring tube, respectively, at its inlet end and at its outlet end.
Besides the opportunity of simultaneously measuring a plurality of such process variables, especially mass flow, density and/or viscosity, by means of one and the same measuring device, a further essential advantage of inline measuring devices with vibration-type measurement-transducers is, among other things, that they exhibit, within predetermined operating limits, a very high measurement accuracy at comparatively low sensitivity to disturbances. Moreover, such a measuring device can be used for practically any flowable or streamable media and can be applied in a multitude of the most different of areas of application of measuring and automation technology.
In the case of inline measuring devices of the described kind, which are applied as Coriolis mass flow meters, the associated measuring device electronics determines, during operation, among other things, a phase difference between the two oscillation measurement signals delivered by the sensor elements and the measuring device electronics issues at its output a measured value signal derived therefrom. The measured value signal represents a measured value corresponding to the time behavior of the mass flow. If, as is usual in the case of such inline measuring devices, the density of the medium is also to be measured, then the measuring device electronics determines therefor, additionally, on the basis of the oscillation measurement signals, an instantaneous oscillation frequency of the measuring tubes. Moreover, also, for example, the viscosity of the medium can be measured by means of the power, especially a corresponding exciter current for the exciter arrangement, needed for maintaining the measuring tube oscillations.
For the operation of the measurement transducer, especially also for the further processing or for the evaluation of the at least one measurement signal, such is, as already indicated, electrically connected with an appropriate measuring device electronics. In industrial measurement and automation technology, this measuring device electronics is, additionally, often connected, via a data transmission system, e.g. a digital data bus, with other measuring devices and/or with a remote, central computer, to which it sends the measured value signals. Example of suitable data transmission systems often include bus systems, especially serial bus systems, such as PROFIBUS-PA, FOUNDATION FIELDBUS or suchlike, along with the corresponding transmission protocols. By means of the central computer, the transmitted, measured-value signals can be processed further and visualized as corresponding measurement results e.g. on monitors and/or they can be converted into control signals for corresponding actuators, such as e.g. solenoid-operated valves, electric motors of pumps, etc. For accommodating the measuring device electronics, such inline measuring devices further include an electronics housing, which can be situated remotely from the measurement transducer and connected therewith only via a flexible line, as proposed e.g. in WO-A 00/36379, or which is, as shown e.g. also in EP-A 1 296 128 or WO-A 02/099363, arranged directly on the measurement transducer, especially on the already the mentioned measurement transducer housing.
In the case of measurement transducers of the described kind, essentially two kinds of tube forms have become established in the market, namely, on the one hand, essentially straight measuring tubes and, on the other hand, measuring tubes curving essentially in a tube plane. Of this latter class, essentially S-, U- or V-shaped tubes are the ones used most often. Especially in the case of Coriolis mass flow measurement transducers serving for measuring mass flows, in the case of both types of tube shapes, for reasons of symmetry, two measuring tubes are used, extending, most often, essentially parallel to one another in the resting state and flowed-through, most often, also in parallel by the medium. Examples in this regard include U.S. Pat. Nos. 4,127,028, 4,768,384, 4,793,191 5,610,342, 5,796,011, or U.S. Pat. No. 6,450,042.
Besides measurement transducers with such double measuring tube arrangements, however, also measurement transducers having a single straight, or curved, measuring tube have long been obtainable in the market. Such vibration-type measurement-transducers with a single measuring tube are described e.g. in U.S. Pat. Nos. 4,524,610, 4,823,614, 5,253,533, 6,006,609, 6,047,457, 6,168,069, 6,314,820, 6,397,685, 6,487,917, 6,516,674, 6,666,098, 6,698,644, 6,711,958, WO-A 03/048693, or the mentioned application DE10354373.2 of the assignee. Each of the measurement transducers shown therein include, among other things: A measuring tube vibrating, at least at times, having an inlet end and an outlet end, and made, for example, of steel, titanium, tantalum or zirconium or corresponding alloys, for conveying the medium to be measured, wherein the measuring tube communicates with a connected pipeline via a first tube segment opening into the inlet end and via a second tube segment opening into the outlet end, in order to enable the medium to flow through the measuring tube, and wherein the measuring tube, during operation, executes mechanical oscillations about an oscillation axis imaginarily connecting the two tube segments; and a, most often, very bending-stiff, tubular or frame-like, support element, for example of steel, for the oscillatable holding of the measuring tube, which is affixed to the first tube segment by means of a first transition piece and to the second tube segment by means of a second transition piece.
For the case described above, wherein the measurement transducer used is one with a single measuring tube, additionally provided in the measurement transducer is a counteroscillator affixed to the measuring tube and suspended oscillatably in the measurement transducer housing. This counteroscillator serves, apart from its function involving the mounting of the oscillation exciter and the sensor elements, for uncoupling, as far as oscillations are concerned, the vibrating measuring tube from the connected pipeline. The counteroscillator, which is most often made of steel, because of the favorable cost thereof, can, in such case, be embodied as a tubular, compensation cylinder, or box-shaped support frame, arranged coaxially to the measuring tube. To the referenced ensemble of features of the individual measurement transducers described above is still to be added, that a straight measuring tube, or straight measuring tubes, as the case may be, is/are, most often, made of pure titanium, a titanium alloy of high titanium content, pure zirconium or a zirconium alloy of high zirconium content, since, compared with measuring tubes of stainless steel, which is likewise possible for straight measuring tubes, usually shorter construction lengths result, and that a curved measuring tube, or curved measuring tubes, as the case may be, is/are especially of stainless steel, although titanium or zirconium, or their alloys, as the case may be, are also, in this instance, possible for the material of the measuring tubes. Beyond this, however, the use of tantalum or corresponding tantalum alloys is, for example, usual as material for the measuring tube, or tubes.
As can be derived, without difficulty, from the above review, practically every measurement transducer of the above-cited state of the art involves at least one, especially bi-, or poly-, metal, interconnected system, which includes a first component—for example, the first or the second end piece—and a second component—for example, the measuring tube—extending at least partially through the first component along an imaginary longitudinal axis of the interconnected system, wherein, usually, the second component contacts with an outer surface in the form of a cylindrical outer surface, flushly, an inner surface of the first component, formed by an inner wall of a bore extending within the first component. Equally, however, also measurement transducers are constructed with double measuring tube arrangements, as especially also described in U.S. Pat. No. 5,610,342, usually of a plurality of such, especially bi-metal, interconnected systems. Besides the interconnected system formed by measuring tube and end piece, other examples of such, especially bi-metal, interconnected systems are cited, especially also the connection of measuring tube and flange, or the connection flange and measurement transducer housing; compare, in this connection, also U.S. Pat. Nos. 6,168,069, 6,352,196, and 6,698,644. Additionally, such a composite system can also be formed, as also described in U.S. Pat. No. 6,047,457, by the affixing of at least one circular, washer-, or disk-, shaped, metal member to the measuring tube between the two end pieces, to serve as a part of the exciter mechanism, or to interact therewith, as the case may be. Beyond this, such metal members can also serve as part of the sensor arrangement or as couplers between the measuring tube and the possibly provided, counteroscillator.
Imposed on the vibration-type measurement-transducers used in industrial measurements and automation technology are very high requirements as regards accuracy of measurement, with such lying, usually, in the range of about 0.1% of the measured value and/or 0.01% of the measured value at the upper end of the range. Required to achieve this is, especially, a very high stability of the zero point, as well as a very high robustness of the delivered measurement signals, especially also in the case of environmental, attachment and/or operating conditions which change significantly during operation. As already discussed in detail in the mentioned U.S. Pat. Nos. 5,610,342, 6,047,457, 6,168,069, 6,519,828, 6,598,281, 6,698,644, 6,769,163, WO-A 03/048693, or the mentioned applications DE102004048765.0 or DE10354373.2 of the assignee, also the mechanical strength, especially the fatigue strength, with which, in such case, the individual components of the aforementioned interconnected systems formed in the measurement transducer are affixed to one another, is assigned a considerable importance. Already the slightest deviation of the strength of the aforementioned interconnected systems from the situation present at calibration can result in significant, no longer manageable fluctuations of the zero point and/or sensitivity and, thus, in practically unusable measurement signals. Usually, such zero point errors attributable to strength-loss phenomena in the interconnected systems can only be eliminated by installation of a new inline measuring device. Particularly the way in which the measuring tube is secured within the outer support element and the possibly present counteroscillator has, in such case, a special influence on the stability of the zero point and/or the availability of the measurement transducer; this subject is also already discussed in detail in U.S. Pat. Nos. 5,610,342, 6,047,457, 6,168,069, 6,598,281, 6,634,241, or WO-A 03/048693.
Traditionally, the components of such interconnected systems are, at least in part, bonded together by soldering, brazing or welding. Thus, for example, it is already discussed in U.S. Pat. No. 4,823,614, that the respective ends of the one measuring tube are inserted in bores of their assigned inlet and outlet, end pieces and secured therein by front and rear welding, soldering or brazing; compare the material beads to be seen in some of the figures. The end pieces are, in turn, secured in the outer support element. Further examples of such interconnected systems with bonded connections are shown in U.S. Pat. Nos. 6,168,069, 6,352,196, 6,519,828, 6,523,421, 6,598,281, 6,698,644, or U.S. Pat. No. 6,769,163, among others.
Especially for the above-described case, in which the first component serves as a coupler between the measuring tube, as second component, and a third component in the form of a counteroscillator, there exists in the manufacture of the measurement transducer, however, to such extent, often a considerable problem in that, depending on the manner of construction and/or on the requirements placed on the measurement transducer on the basis of its application, at least two components are to be connected together oscillation-resistantly, which are made of different metals, for example steel and titanium. In bi-metal, interconnected systems, thus those in which at least the first component and the second component are made of different metals, it is, unfortunately, not always possible, without more, to prevent, with certainty, strength-loss in the interconnections. As can be perceived, for example, from U.S. Pat. Nos. 6,047,457, 6,168,069, 6,352,196, 6,598,281, 6,634,241, 6,523,421 or 6,698,644, namely, problems can arise in the case of such bi-metal, interconnected systems with regard to the long-term strength of the solder, or braze, connections which had to be used because of the lacking weldability. Such problems can be attributed to, among other things, insufficient wetting and/or radially alternating, mechanical loading of the joints. This last is especially attributable to the, in part, significantly differing thermal expansions of the components, be it during manufacture or during operation. A further problem of such bonded, solder or braze connections is also the material-degrading, oscillatory wear in the area of the joints, pointed out in U.S. Pat. Nos. 6,519,828, or 6,598,281.
A possibility for reducing this risk of strength-loss in interconnected systems formed, for example, by a measuring tube of a Coriolis mass flow measurement transducer and a metal member pushed thereon and affixed thereto can be provided, according to U.S. Pat. No. 6,698,644, by having the components introduce additional compressive stresses into the solder, or braze, connection, whereby the preferably large-area, solder or braze connection between the components can be stabilized. A further possibility for improving the longterm strength of such interconnected systems lies in affixing the components together using press joints. Thus, in the already mentioned U.S. Pat. No. 5,610,342, as well as in WO-A 03/048693, in each case, a securement method is proposed for measuring tubes in end pieces, wherein each end of the measuring tube is inserted into a corresponding bore of an inlet end piece, or an outlet end piece, as the case may be, and pressure is applied to the inner wall of the bore, especially in the absence of heating, by means of a roller tool placed in the end, whereby a high-strength, frictionally interlocking connection is formed between the first and second components. A roller tool appropriately suited for this method is, for example, also described in U.S. Pat. No. 4,090,382, in the context of a method of manufacturing boilers or heat exchangers. A further possibility for manufacturing such interconnected systems formed by means of high-strength, frictionally interlocking connections is, as e.g. also proposed in U.S. Pat. No. 6,047,457, to compress the first component, after it has been pushed, or slipped, onto the second component, by means of a pressing tool, so that the first component undergoes mixed plastic-elastic deformation below a recrystallization temperature of the material of the component, especially at room temperature. The deformation forces applied for this purpose are, in such case, always so developed, that the second component experiences essentially no cross sectional reduction and/or narrowing, so that an initial inner diameter of the second component remains practically unchanged throughout, after manufacture of the interconnected system. A suitable apparatus for the pressing is shown, for example, in U.S. Pat. No. 3,745,633. Alternatively to the plastic-elastic pressing, such an interconnected system formed by means of frictional interlocking can also be produced, for example, by, as also shown in U.S. Pat. Nos. 6,598,281 or 6,519,828, thermally shrinking the first component onto the second component or by clamping the first component with the second component using elastically deformable clamping elements.
Going further, U.S. Pat. Nos. 6,598,281, or 6,519,828 disclose that, however, even in the case of purely frictionally interlocking, press connections, it is not always certain, due to oscillatory wear, that possible loss of strength of the interconnected system can be avoided. Moreover, such oscillatory wear of the materials of the interconnected system can lead to corrosion in the area of the mutually contacting surfaces. Furthermore, as can be perceived from WO-A 03/048693, the usually different expansion behaviors of the components of the above interconnected systems, thus for example the above-mentioned end pieces and the tube segments of the measuring tube, in each case, clamped therein, can lead, in the case of temperature fluctuations, especially in the case of possible temperature shocks, such as can arise e.g. in the case of execution of regularly scheduled cleaning measures with extremely hot rinsing liquids, to a sinking of the clamping forces exerted by the first component on the second component below a critical level. This, in turn, can mean that, due to thermally related expansions, the first component and the second component can locationally lose the mechanical contact brought about by the rolling, pressing or shrinking, so that the interconnected system loses strength to an unacceptable degree. As a result off this, in turn, the pull-out strength of the interconnected system can sink, and, to such extent, also, with press joints of such type, the required high zero-point stability of the measurement transducer can not be assured, in the absence of extra measures. For overcoming the deficiency in interconnected systems of the described kind caused by oscillatory wear between the components, it is proposed in U.S. Pat. Nos. 6,598,281 or 6,519,828, as the case may be, additionally to bond the pertinent components together, especially with use of a fill material serving as an intermediate layer, following manufacture of the press-joint, but this raises anew the possibility of the above-mentioned problems associated with soldered or brazed connections. Accordingly, in WO-A 03/048693, an interconnected system is proposed, which is given an increased twist strength, by the measure of forming in the inner wall of the first component a groove running in the direction of the longitudinal axis of the interconnected system. This groove can effectively prevent rotation of the first component relative to the second component by the formation of a mechanical interlocking acting in a circumferential direction. However, also this interconnected system, as proposed, can experience a decrease in its nominal pull-out strength, be it because of oscillatory wear and/or thermally related expansion.